Virtually from the beginning of recorded history man has required the means to determine true horizontal and vertical in major construction of all types. This need has continued undiminished to the present time, and in fact the primitive carpenter's level has persevered virtually unchanged for many years. In such a level a vial or generally cylindrical container is partially filled with a liquid so a bubble of air remains. When the vial is placed horizontally on a level surface, i.e., when the axis of the vial is coparallel with the local horizon, the bubble will be equidistant from the ends of the cylinder, i.e., it will be centered. Any deviation from level will be manifested by deviations of the bubble from centrality, i.e., it will be closer to one end than the other. In this application, horizontal level will mean a line coparallel to the local horizon, and vertical level will mean a line perpendicular to the local horizon.
The classical bubble level has limitations in accuracy as well as convenience. Because it relies on visual sighting when used in, e.g., carpentry the user's eyes may need to continually shift between the level and the focus of his work, such as the end of a beam. Consequently, there is a need for a level which can be used by one person without attendant constant close eye contact. It is desirable that such a level be responsive, both in the sense of a rapid response time to changes in horizontal position, and in the sense of having the capability of detecting small deviations from absolute level. Additionally it is paramount that such a level be sturdy, inexpensive, simple to manufacture, and convenient to use under extremes of field conditions such as temperature, humidity, and cramped work space.
Although there are examples of levels and level sensing units based on electroptical devices capable of great precision and accuracy such articles are excluded from further consideration here because of their cost and sometimes because of their inconvenience in operation. The construction worker, for example, needs a much simpler device. Perhaps the best example of an attempt to fill this need is that described in U.S. Pat. No. 4,152,839, which utilizes a sealed capsule containing a bead of mercury which acts much like an air bubble in a vial of liquid. When in a horizontal level position mercury, an electrical conductor, is at the center and in electrical contact with electrodes which energize a current indicating a level status, e.g., a buzzer or bulb. When in a position deviating from horizontal the bead of mercury tends to roll away from the center, breaking electrical contact and deenergizing the circuit. The many possible variations of this theme need not be elaborated on at this time.
The above design is essentially a mercury-actuated switch. An acknowledged limitation of such a switch is its relative sluggish response and insensitivity to small changes, both arising because the extremely high surface tension of mercury requires a relatively large degree of angular change from level for movement of the mercury bead. Although somewhat of an exaggeration, it is almost as if such a mercury-actuated switch exhibits a step response rather than a continual one to changes in angular position of the mercury-containing capsule.
I have devised a simple, yet accurate and inexpensive level sensing unit with a very rapid response time which can readily drive, or be incorporated into, an electrical circuit indicating the state of level and which can be readily incorporated into, e.g., a contractor's level which is rugged, of universal utility, and can be used quite conveniently without eye contact. In one aspect my invention is a level sensing unit as will be described. In another aspect my invention is a level, as for use by carpenters, plumbers, and other tradesman in the construction industry, incorporating the level sensing unit in a simple electrical circuit so as to give audible and/or optical indications of level and/or deviations therefrom.